Choline acetyltransferase catalyzes the biosynthesis of the neurotransmitter substance, acetylcholine, according to the following chemical equation: Acetyl-CoA plus Choline yields (reversibly) Acetylcholine plus CoA. The soluble enzyme isolated in bovine brain contains an active-site sulfhydryl group. This group reacts with acetyl-CoA to form an acetyl- thioenzyme intermediate and CoA. The CoA is rather tightly, but not covalently bound to the acetyl-enzyme. Choline attacks the acetyl-enzyme in the rate-limiting portion of the reaction forming acetylcholine, CoA and the regenerated enzyme. It is proposed to characterize and identify this active-site sulfhydryl group. Evidence with ethoxyformic anhydride implicate a histidine residue in the active site. Glutamate decarboxylase catalyzes the biosynthesis of gamma-aminobutyric acid (an alleged neurotransmitter substance) and CO2. Studies with inhibitor substances are planned to characterize the active site. Attempts to resolve the enzyme into apoenzyme and pyridoxal phosphate are planned.